To improve fuel efficiency of cars, various parts have been conventionally expected to reduce the weight. An aluminum alloy is used instead of cast iron as a material for an engine block, and a magnesium alloy is used instead of steel as a material for an engine cover and an oil pan, for example.
When the parts made of the aluminum alloy or the magnesium alloy described above are fastened with a conventional steel bolt, the steel bolt is likely to loosen because of large difference between the coefficient of linear expansion of the aluminum alloy and the magnesium alloy and that of the steel. Furthermore, the steel bolt is likely to be corroded by contact with different types of metals. To sufficiently secure the reliability of fastening, it is necessary to make a tapped hole in the parts deeper and make the length of the shaft of the steel bolt longer. Furthermore, it is necessary to make the diameter of the steel bolt larger. The length of the shaft of the steel bolt, however, affects the thickness of the parts, and the diameter of the steel bolt affects the width of a flange in which the tapped hole is formed. As a result, the use of the steel bolt to fasten the parts made of the aluminum alloy and the magnesium alloy is an obstacle to weight reduction.
To solve such a problem of the steel bolt, there have been developed techniques for employing an aluminum alloy bolt as a fastening member that fastens parts made of an aluminum alloy and a magnesium alloy (refer to Patent Literature 1, for example). Patent Literature 1 discloses a 6000 series aluminum alloy as a material for a bolt having excellent workability when being processed into a bolt and sufficient strength. The aluminum alloy bolt has small difference in the coefficient of liner expansion with that of the aluminum alloy and the magnesium alloy constituting various parts. Furthermore, the aluminum alloy bolt is less likely to be corroded by contact with different types of metals. With these properties, the aluminum alloy bolt can secure the reliability of fastening even if the tapped hole in the parts is made shallower or the diameter of the bolt is made smaller. The aluminum alloy bolt is thus suitably used to reduce the weight.
There have also been developed a 7000 series aluminum alloy as an aluminum alloy having higher strength than that of the 6000 series aluminum alloy (refer to Patent Literature 2, for example). The use of the 7000 series aluminum alloy enables production of a bolt having higher strength than that of the 6000 series aluminum alloy bolt.